The present invention relates generally to radio communication systems and, more specifically, to a multiple-conversion receiver where the final intermediate frequency (IF) stage operates at a higher frequency than the previous IF stage.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices. To this end, the design of receivers has received considerable attention.
In a conventional receiver, band pass filters are used to reduce noise and interference. Sometimes, additional filtering may be desired and a dual down conversion superheterodyne receiver may be used. As illustrated in FIG. 1, the dual down conversion superheterodyne receiver 30 receives signals over an antenna 31. The received signal is then filtered in an RF filter 32 and amplified and mixed in a low noise amplifier/mixer 34 with a signal from the first local oscillator 36 to generate a first IF.
The first IF signal is then filtered in the first IF filter 38. In cellular radio applications, for example, the first IF filter 38 is typically a passive crystal filter. In superheterodyne receivers a second IF stage 40 is provided for the second down conversion. Thus, the signal output by the first IF filter 38 is then amplified in an IF filter/mixer 42. The amplified signal is then mixed with a signal from a second local oscillator 44 to mix the signal down to a second IF. The second IF signal is then filtered in a second IF filter 46, typically a passive ceramic filter, and is output to an amplifier 48. This signal is then processed by detector 50 to detect the information symbols within the received signal using techniques that are well known to those skilled in the art.
The design of multiple-conversion superheterodyne receivers has traditionally followed two principles: (1) the conversion should result in progressively lower intermediate frequencies--for example, a 100 MHz signal might be converted to a first IF at 10.7 MHz and then to a second IF at 455 KHz; and (2) the center frequency of the second IF should be as high as possible, commensurate with filter-performance limitations and adjacent-channel suppression requirements, in order to minimize the demand on the first IF filter to suppress mixing products that result from the second conversion and due to manufacturing constraints associated with the materials (e.g. crystal or ceramic) used to construct these filters.
Recent developments have made the second of these principles obsolete, particularly the replacement of passive IF filters by the modern active IF filter. Passive filters used in these conventional receiver structures are considered problematic because, for example, they are expensive and require a large mounting area. Active filtering provides an attractive alternative to this problem. For example, the active filter circuits can be integrated along with other IF circuits, e.g., mixers, voltage controlled oscillators (VCO) and detectors, to create a more compact receiver structure. As described below and mainly to optimize the performance of the active filter and to reduce its cost, it is advantageous to minimize the center frequency of the second IF filter when using an active filter in the second down conversion stage.
In this regard, note that the dynamic range of an active filter is related to the quality factor (Q). As Q increases, the dynamic range goes down (see Gert Groenewold, The Design of High Dynamic Range Continuous-Time Integratable Bandpass Filters, IEEE vol. 38, August 1991). In addition, as Q increases the component spread in the filter structure also increases leading to a problem with parasitic capacitance. As the parasitic capacitance increases, the total capacitance used must increase to scale the impedance of the filter resulting in the requirement of more current. At high center frequencies, the parasitic capacitance changes the filter's pole positions and lowers the filter's tolerance to an impractical level. Therefore, the center frequency for the second IF should be low in order to produce a practical filter for integration in silicon.
At the same time, however, a higher second IF is needed for processing by baseband components such as detector 50. There are alternatives available for resolving the tension between the desire to use a lower second IF for active filter design and the need for a higher second IF for baseband processing. One alternative is to redesign the baseband circuits so that they will function at lower IFs. However, redesigning the baseband circuits to operate at lower IFs would be time consuming and expensive. Another alternative is to design a higher IF active filter. However, designing an active filter for higher IF results in higher current and increased silicon area.
Therefore, there exists a need for an apparatus which overcomes the aforedescribed limitations and resolves the need for both lower and higher second IFs in a cost effective manner.